The Republican Presidential candidate Donald Trump has intensified attacks against rivals Ted Cruz (Senator from Texas) and John Kasich (Governor of Ohio State) on Monday after they announced a strategic alliance to stop (an attempt) the New York billionaire from winning the party's U.S. presidential nomination.

Both Cruz and Kasich’s campaigns released statements on Sunday saying that Cruz will focus his campaign resources on winning enough delegates in Indiana while Kasich will focus his efforts on western states including Oregon and New Mexico.

"To ensure that we nominate a Republican who can unify the Republican Party and win in November, our campaign will focus its time and resources in Indiana and in turn clear the path for Governor Kasich to compete in Oregon and New Mexico, and we would hope that allies of both campaigns would follow our lead," Cruz's campaign manager Jeff Roe said in a statement.

Kasich's campaign also issued a similar statement.

"Due to the fact that the Indiana primary is winner- takes-all statewide and by congressional district, keeping Trump from winning a plurality in Indiana is critical to keeping him under 1,237 bound delegates before Cleveland," said John Weaver, chief strategist for Kasich for America.

Trump is expected to sweep the five states holding primary elections on Tuesday, prompting Cruz and Kasich to announce their unusual tactic of agreeing not to compete against each other in Indiana, New Mexico and Oregon.

In a sign that Cruz has no intention of backing out of the race before the July convention in Cleveland, his campaign manager Jeff Roe posted on Twitter on Monday that they had narrowed the search for a vice presidential running mate to a "short list" and were vetting potential candidates.

Trump, the current front-runner, needs 1,237 delegates to win the Republican nomination. If he falls short, the Republican convention in July will evolve into a rarest contested convention.

Trump has repeatedly denounced the system, saying he should win the nomination even if he falls slightly short of the majority — something officials with the Republican National Committee have ruled out.

Earlier in Philadelphia, Kasich tried to play down the alliance with Cruz as simply a way for him to save money by not campaigning in certain areas. He said he was not asking supporters in those places not to vote for him. "So what? What's the big deal?" Kasich said. "I'm not over there campaigning and spending resources. We have limited resources." Trump has dominated the nominating contests so far but still faces a tough path to earn the delegates needed to lock up the nomination before the convention.

Traditionally, candidates wait until they have secured the nomination to select a running mate. Trump denounced the move by his opponents to band together as disgusting and compared it to illegal collusion in stock trading. "It shows how weak they are. It shows how pathetic they are," Trump told a crowd in Warwick, Rhode Island.

Trump said the Cruz-Kasich deal bolstered his contention that the Republican system for choosing delegates is rigged. Party officials have said the rules have long been known. If no candidate has enough delegate support on the first vote at the national convention, many delegates can switch to another candidate on subsequent ballots. The move by Kasich and Cruz could be motivated by simple math - it is virtually impossible for either to get enough delegates to win the nomination. "A lot of observers were urging them to do this weeks or months ago, right after Rubio dropped out," Sabato said.